RNA interference refers to the process of sequence-specific post-transcriptional gene silencing in animals that is mediated by small inhibitory nucleic acid molecules (siRNAs) a double-stranded RNA (dsRNA) that is homologous in sequence to a portion of a targeted messenger RNA. See Fire, et al., Nature 391:806, 1998, and Hamilton, et al., Science 286:950-951, 1999. These dsRNAs serve as guide sequences for the multi-component nuclease machinery within the cell that degrade the endogenous-cognate mRNAs (i.e., mRNAs that share sequence identity with the introduced dsRNA).
The process of post-transcriptional gene silencing is thought to be an evolutionarily-conserved cellular defense mechanism used to prevent the expression of foreign genes and is commonly shared by diverse flora and fauna. Fire, et al., Trends Genet. 15:358, 1999. Such protection from foreign gene expression may have evolved in response to the production of double-stranded RNAs (dsRNAs) derived from viral infection or from the random integration of transposon elements into a host genome via a cellular response that specifically destroys homologous single-stranded RNA or viral genomic RNA.
RNAi has been studied in a variety of systems. Fire et al. were the first to observe RNAi in C. elegans. Nature 391:806, 1998. Bahramian & Zarbl and Wianny & Goetz describe RNAi mediated by dsRNA in mammalian systems. Molecular and Cellular Biology 19:274-283, 1999, and Nature Cell Biol. 2:70, 1999, respectively. Hammond, et al., describes RNAi in Drosophila cells transfected with dsRNA. Nature 404:293, 2000. Elbashir, et al., describe RNAi induced by introduction of duplexes of synthetic 21-nucleotide RNAs in cultured mammalian cells including human embryonic kidney and HeLa cells. Nature 411:494, 2001.
To date, siRNA is an emerging novel field with significant clinical implications. However, the technology is hampered by a number of limitations, such as difficulty and impracticality of its delivery in vivo. Although viral vector-based siRNA delivery systems have been widely used, their specificity and safety remains significant issue. While delivery of nucleic acids offers advantages over delivery of cytotoxic proteins such as reduced toxicity prior to internalization, there is a need for high specificity of delivery, which is currently unavailable with the present systems.
In view of some of the problems associated with gene therapy, there is a need for improved treatments which are more effective and are not associated with such disadvantages.